Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017
Balance at April 30, 2017			$ 1,370
Retirement of treasury stock			0
Net income	$ 190	$ 182	607	$ 524
Net other comprehensive income (loss)	9	$ (26)	8	$ (83)
Cash dividends			(773)
Acquisition of treasury stock			(1)
Stock-based compensation expense			14
Stock issued under compensation plans			26
Loss on issuance of treasury stock issued under compensation plans			(50)
Stock split			0
Balance at January 31, 2018	1,201		1,201
Additional Paid-in Capital [Member]
Balance at April 30, 2017			65
Retirement of treasury stock			(8)
Stock-based compensation expense			14
Loss on issuance of treasury stock issued under compensation plans			(50)
Stock split			(14)
Balance at January 31, 2018	7		7
Retained Earnings [Member]
Balance at April 30, 2017			4,470
Retirement of treasury stock			(2,684)
Net income			607
Cash dividends			(773)
Balance at January 31, 2018	1,620		1,620
AOCI Attributable to Parent [Member]
Balance at April 30, 2017			(390)
Net other comprehensive income (loss)			8
Balance at January 31, 2018	(382)		(382)
Treasury Stock, Common [Member]
Balance at April 30, 2017			(2,843)
Retirement of treasury stock			2,702
Acquisition of treasury stock			(1)
Stock issued under compensation plans			26
Balance at January 31, 2018	(116)		(116)
Common stock, Class A, voting [Member] | Common Stock [Member]
Balance at April 30, 2017			25
Stock split			0
Balance at January 31, 2018	25		25
Common stock, Class B, nonvoting [Member] | Common Stock [Member]
Balance at April 30, 2017			43
Retirement of treasury stock			(10)
Stock split			14
Balance at January 31, 2018	$ 47		$ 47